How to rotate a PDF
Rotating a PDF to read it and rotating it permanently are two different things. Here is how to make the fix stick.
Why the rotation doesn't save
Most PDF readers, including the ones built into iPhone, Android, Mac and Windows, let you rotate the current view with a button or gesture so a sideways page reads right-side up. That rotation is just how the viewer is displaying the page in that session; it is not written back into the file, so the page opens sideways again next time, and anyone else who opens the PDF sees it unrotated too.
On a Mac with Preview
Preview can rotate a page and save it that way: select the page in the sidebar, use the rotate buttons in the toolbar (or Tools, Rotate Left/Right), then save the file. This is one of the few built-in viewers that actually writes the rotation into the saved PDF rather than only changing the display.
On Windows, iPhone or Android
None of the default PDF viewers on these platforms (Edge's reader, or the Files/Photos apps on iPhone and Android) save a rotation permanently; whatever rotate control they offer only affects that viewing session. This is a genuine gap, not a hidden setting, which is why the fix tends to come up as a repeated search across every platform except Mac.
A tool that rotates and saves it for good
Stayput's rotate PDF tool rotates all pages or just the ones you select by 90, 180 or 270 degrees and saves the change permanently into a new file, directly in the browser without uploading the document. It works the same way in any modern browser, including on Windows, iPhone and Android where there is no built-in equivalent.
Questions
Why does my PDF keep opening sideways even after I rotate it?
Most viewers only rotate the on-screen display for that session; the rotation is not saved into the file unless the tool specifically supports saving it, like Preview on a Mac or a dedicated rotate tool.
Can I rotate just one page instead of the whole document?
Yes, both Preview and a dedicated PDF rotation tool let you select individual pages to rotate rather than applying it to every page.
Does rotating a PDF affect its text or image quality?
No, rotation only changes the page's orientation; the underlying text and images are unchanged.
Why are scanned PDFs so often sideways in the first place?
A scanner or phone camera can capture a page in whatever orientation it was fed or held, and the scanning software doesn't always auto-correct it, leaving the saved PDF rotated.